An excerpt from
http://www.westonaprice.org/basicnutrition/vitamindmiracle.html
  The ultraviolet wavelength that stimulates our 
bodies to produce vitamin D is UV-B. It is 
sometimes called the "burning ray" because it is 
the primary cause of sunburn (erythema). However, 
UV-B initiates beneficial responses, stimulating 
the production of vitamin D that the body uses in 
many important processes. Although UV-B causes 
sunburn, it also causes special skin cells called 
melanocytes to produce melanin, which is 
protective. UV-B also stimulates the production 
of Melanocyte Stimulating Hormone (MSH), an 
important hormone in weight loss and energy 
production.
  The reason it is difficult to get adequate 
vitamin D from sunlight is that while UV-A is 
present throughout the day, the amount of UV-B 
present has to do with the angle of the sun's 
rays. Thus, UV-B is present only during midday 
hours at higher latitudes, and only with 
significant intensity in temperate or tropical 
latitudes. Only 5 percent of the UV-B light range 
goes through glass and it does not penetrate 
clouds, smog or fog.

Sun exposure at higher latitudes before 10 am or 
after 2 pm will cause burning from UV-A before it 
will supply adequate vitamin D from UV-B. This 
finding may surprise you, as it did the 
researchers. It means that sunning must occur 
between the hours we have been told to avoid. 
Only sunning between 10 am and 2 pm during summer 
months (or winter months in southern latitudes) 
for 20-120 minutes, depending on skin type and 
color, will form adequate vitamin D before 
burning occurs.9

It takes about 24 hours for UV-B-stimulated 
vitamin D to show up as maximum levels of vitamin 
D in the blood. Cholesterol-containing body oils 
are critical to this absorption process.10 
Because the body needs 30-60 minutes to absorb 
these vitamin-D-containing oils, it is best to 
delay showering or bathing for one hour after 
exposure. The skin oils in which vitamin D is 
produced can also be removed by chlorine in 
swimming pools.

The current suggested exposure of hands, face and 
arms for 10-20 minutes, three times a week, 
provides only 200-400 IU of vitamin D each time 
or an average of 100-200 IU per day during the 
summer months. In order to achieve optimal levels 
of vitamin D, 85 percent of body surface needs 
exposure to prime midday sun. (About 100-200 IU 
of vitamin D is produced for each 5 percent of 
body surface exposed, we want 4,000 iu.) Light 
skinned people need 10-20 minutes of exposure 
while dark skinned people need 90-120 minutes.11

Latitude and altitude determine the intensity of 
UV light. UV-B is stronger at higher altitudes. 
Latitudes higher than 30° (both north and south) 
have insufficient UV-B sunlight two to six months 
of the year, even at midday.12 Latitudes higher 
than 40° have insufficient sunlight to achieve 
optimum levels of D during six to eight months of 
the year. In much of the US, which is between 30° 
and 45° latitude, six months or more during each 
year have insufficient UV-B sunlight to produce 
optimal D levels. In far northern or southern 
locations, latitudes 45° and higher, even summer 
sun is too weak to provide optimum levels of 
vitamin D.13-15